In order to develop small, thin information devices having high integration and high performance, it is necessary to further develop a fine processing technique for electronic circuits and the like, and to provide a technique of fine processing at low cost and low energy. A technique which has recently attracted attention as a solving method is to form, at low cost and low energy, fine conductive wiring by using an ink using metal nanoparticles as a conductive material and printing with the ink. That is, because metal fine nanoparticles have high surface energy and thus cause a fusion temperature depression on particle surfaces, conductivity equivalent to that of bulk silver can be obtained by heating a coating film or printed matter at a low temperature of about 150° C., and fine wiring can be drawn on a substrate with low heat resistance, such as PET or the like. Therefore, the metal fine nanoparticles have a high utility value.
The inventors have already reported a metal nanoparticle dispersion including metal nanoparticles contained in a dispersion, which is formed by a polymer compound containing a polyalkyleneimine chain, a hydrophilic segment, and a hydrophobic segment in a solvent, and also reported a method for producing the metal nanoparticle dispersion (refer to, for example, Patent Literature 1). The metal nanoparticle dispersion is a dispersion formed by reducing metal ions by means of the strong reducing ability, coordinate bonding force, and electrostatic interaction of the polyalkyleneimine chain in the polymer compound used, and fixing the metal as nanoparticles in the dispersion. Even when the morphology of the dispersion is changed with contraction or the like of the polyalkyleneimine chain due to the specific functions of the polyalkyleneimine chain, the hydrophilic segment and the hydrophobic segment in the polymer compound exhibit excellent self-organization ability due to high affinity for the solvent and strong associative force by the interaction between the segments. Therefore, a stable dispersion state is maintained over a long period of time in the solvent without deterioration in dispersion stability as the dispersion.
Also, a formed product using the metal nanoparticle dispersion is reported (refer to, for example, Patent Literature 2). The formed product can be produced by using the metal nanoparticle dispersion as it is without the need for a complicated operation, but electric characteristics are unsatisfactory. For example, a resistance value of only 7.8×10−4 Ω·cm is shown even by heat treatment at 200° C. for 30 minutes, and the resistance value is as high as a value of two-digit higher than 1.6×10−6 Ω·cm of bulk silver. In addition, the requirement for high-temperature heat treatment makes it difficult to apply to a plastic substrate used for a flexible display and the like, and the lack of versatility causes the need for improvement.
In this situation, the inventors found and reported that a formed product having any desired shape and exhibiting high conductivity in low-temperature sintering without complicated treatment or the like can be produced by applying, on a substrate, a silver paste having a fusion temperature within a specified range and containing a silver nanoparticle dispersion, which is prepared by using a compound containing a polyethylene imine skeleton as a protective agent, and a compound having a functional group reactable with nitrogen atoms in the polyethyleneimine (refer to, for example, Patent Literature 3).